On Mon, 18 May 2026 at 13:43 +0200, Tomasz Kamiński wrote:
This implements changes section 4.6 of P3016R6, and initializer_list related
parts of 4.7. The change makes immediately dangling invocations of std::begin,
std::end, and std::data on braced-init list ill-formed (see range_access_neg.cc
and range_access17_neg.cc):
auto it = std::begin({1, 2, 3}); // ILL-FORMED, it was dangling
(it == std::end({1, 2, 3})); // ILL-FORMED, was unspecified
auto* ptr = std::data({1, 2, 3}); // ILL-FORMED, ptr was dangling
However, similary problemetic calls for std::rbegin, std::rend remain
well-formed (see range_access14_neg.cc), as initializer_list overloads
are preserved for these functions:
auto rit = ranges::rbegin({1, 2, 3}); // COMPILES, dangling
auto rend = ranges::rend({1, 2, 3}); // COMPILES, danging
Note, that non-problematic std::size({1, 2, 3}) and std::empty({1, 2, 3})
use c-array overloads, and remain well-formed.
Per paper, to keep std::data(il) and std::empty(il) well-formed, the data
and empty member are added to initializer_list.
libstdc++-v3/ChangeLog:
* include/bits/version.def (initializer_list): Define with value
202511 for C++26.
* include/bits/version.h: Regenerate.
* libsupc++/initializer_list (initializer_list::data)
(initializer_list::empty) [__glibcxx_initializer_list >= 202511L]:
Define.
(std::begin(initializer_list<_Tp>), std::end(initializer_list<_Tp>)):
Define only if __glibcxx_initializer_list < 202511L (i.e. not defined).
* include/bits/range_access.h (std::empty(initializer_list<_Tp>))
(std::data(initializer_list<_Tp>)): Define only if
__glibcxx_initializer_list < 202511L (i.e. not defined).
* testsuite/18_support/initializer_list/range_access.cc: Move test for
brace-init list to range_access_neg.c. Included <iterator> in C++26 or
later mode.
* testsuite/18_support/initializer_list/data_empty_mem.cc: New test.
* testsuite/18_support/initializer_list/range_access14.cc: New test.
* testsuite/18_support/initializer_list/range_access14_neg.cc: New test.
* testsuite/18_support/initializer_list/range_access17.cc: New test.
* testsuite/18_support/initializer_list/range_access17_neg.cc: New test.
* testsuite/18_support/initializer_list/range_access_neg.cc: New test.
---
I was experimenting with backporting the change up to C++11, as a fix for
always dangling std::begin({....}), but found out that std::begin(il) (for il
being initializer_list) becomes non-constexpr until C++17. This could be
addressed by backporting constexpr on std::begin(Container). But the paper
does not fully resolve the issue: std::rbegin({...}) is still well-formed.
Making this C++26 change then. We could still backport data/empty members,
but waiting to see responses for other implementers.
Testing on x86_64-linux. OK for trunk when all tests passes?
OK
libstdc++-v3/include/bits/range_access.h | 4 ++++
libstdc++-v3/include/bits/version.def | 9 +++++++
libstdc++-v3/include/bits/version.h | 10 ++++++++
libstdc++-v3/libsupc++/initializer_list | 13 ++++++++++
.../initializer_list/data_empty_mem.cc | 21 ++++++++++++++++
.../initializer_list/range_access.cc | 10 +++-----
.../initializer_list/range_access14.cc | 24 +++++++++++++++++++
.../initializer_list/range_access14_neg.cc | 18 ++++++++++++++
.../initializer_list/range_access17.cc | 16 +++++++++++++
.../initializer_list/range_access17_neg.cc | 14 +++++++++++
.../initializer_list/range_access_neg.cc | 13 ++++++++++
11 files changed, 145 insertions(+), 7 deletions(-)
create mode 100644
libstdc++-v3/testsuite/18_support/initializer_list/data_empty_mem.cc
create mode 100644
libstdc++-v3/testsuite/18_support/initializer_list/range_access14.cc
create mode 100644
libstdc++-v3/testsuite/18_support/initializer_list/range_access14_neg.cc
create mode 100644
libstdc++-v3/testsuite/18_support/initializer_list/range_access17.cc
create mode 100644
libstdc++-v3/testsuite/18_support/initializer_list/range_access17_neg.cc
create mode 100644
libstdc++-v3/testsuite/18_support/initializer_list/range_access_neg.cc
diff --git a/libstdc++-v3/include/bits/range_access.h
b/libstdc++-v3/include/bits/range_access.h
index 5a748257f19..b89129f0233 100644
--- a/libstdc++-v3/include/bits/range_access.h
+++ b/libstdc++-v3/include/bits/range_access.h
@@ -328,6 +328,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
empty(const _Tp (&)[_Nm]) noexcept
{ return false; }
+#if __glibcxx_initializer_list < 202511L
/**
* @brief Return whether an initializer_list is empty.
* @param __il Initializer list.
@@ -338,6 +339,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
constexpr bool
empty(initializer_list<_Tp> __il) noexcept
{ return __il.size() == 0;}
+#endif
/**
* @brief Return the data pointer of a container.
@@ -374,6 +376,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
data(_Tp (&__array)[_Nm]) noexcept
{ return __array; }
+#if __glibcxx_initializer_list < 202511L
/**
* @brief Return the data pointer of an initializer list.
* @param __il Initializer list.
@@ -384,6 +387,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
constexpr const _Tp*
data(initializer_list<_Tp> __il) noexcept
{ return __il.begin(); }
+#endif
#endif // __glibcxx_nonmember_container_access
#ifdef __glibcxx_ssize // C++ >= 20
diff --git a/libstdc++-v3/include/bits/version.def
b/libstdc++-v3/include/bits/version.def
index 1f0d3a2670e..efcc0852af5 100644
--- a/libstdc++-v3/include/bits/version.def
+++ b/libstdc++-v3/include/bits/version.def
@@ -2410,6 +2410,15 @@ ftms = {
};
};
+ftms = {
+ name = initializer_list;
+ values = {
+ v = 202511;
+ cxxmin = 26;
+ };
+};
+
+
// Standard test specifications.
stds[97] = ">= 199711L";
stds[03] = ">= 199711L";
diff --git a/libstdc++-v3/include/bits/version.h
b/libstdc++-v3/include/bits/version.h
index 66ac0ebef68..9402f25df37 100644
--- a/libstdc++-v3/include/bits/version.h
+++ b/libstdc++-v3/include/bits/version.h
@@ -2675,4 +2675,14 @@
#endif /* !defined(__cpp_lib_is_structural) */
#undef __glibcxx_want_is_structural
+#if !defined(__cpp_lib_initializer_list)
+# if (__cplusplus > 202302L)
+# define __glibcxx_initializer_list 202511L
+# if defined(__glibcxx_want_all) || defined(__glibcxx_want_initializer_list)
+# define __cpp_lib_initializer_list 202511L
+# endif
+# endif
+#endif /* !defined(__cpp_lib_initializer_list) */
+#undef __glibcxx_want_initializer_list
+
#undef __glibcxx_want_all
diff --git a/libstdc++-v3/libsupc++/initializer_list
b/libstdc++-v3/libsupc++/initializer_list
index baf47baa5f8..fbea49dfb01 100644
--- a/libstdc++-v3/libsupc++/initializer_list
+++ b/libstdc++-v3/libsupc++/initializer_list
@@ -40,6 +40,9 @@
#include <bits/c++config.h>
+#define __glibcxx_want_initializer_list
+#include <bits/version.h>
+
namespace std _GLIBCXX_VISIBILITY(default)
{
/// initializer_list
@@ -77,8 +80,17 @@ namespace std _GLIBCXX_VISIBILITY(default)
// One past the last element.
constexpr const_iterator
end() const noexcept { return begin() + size(); }
+
+#if __glibcxx_initializer_list >= 202511L
+ constexpr bool
+ empty() const noexcept { return _M_len == 0; }
+
+ constexpr const value_type*
+ data() const noexcept { return _M_array; }
+#endif
};
+#if __glibcxx_initializer_list < 202511L
/**
* @brief Return an iterator pointing to the first element of
* the initializer_list.
@@ -100,6 +112,7 @@ namespace std _GLIBCXX_VISIBILITY(default)
constexpr const _Tp*
end(initializer_list<_Tp> __ils) noexcept
{ return __ils.end(); }
+#endif // __glibcxx_initializer_list < 202511L
}
#endif // C++11
diff --git
a/libstdc++-v3/testsuite/18_support/initializer_list/data_empty_mem.cc
b/libstdc++-v3/testsuite/18_support/initializer_list/data_empty_mem.cc
new file mode 100644
index 00000000000..d5673ebf592
--- /dev/null
+++ b/libstdc++-v3/testsuite/18_support/initializer_list/data_empty_mem.cc
@@ -0,0 +1,21 @@
+// { dg-do compile { target c++26 } }
+
+#include <initializer_list>
+#include <iterator>
+
+#ifndef __cpp_lib_initializer_list
+# error "Feature-test macro for text_encoding missing in <initializer_list>"
+#elif __cpp_lib_initializer_list != 202511L
+# error "Feature-test macro for text_encoding has wrong value in
<initializer_list>"
+#endif
+
+void
+test02()
+{
+ static constexpr std::initializer_list<int> il{1};
+ static_assert( il.data() == il.begin() );
+ static_assert( il.empty() == false );
+ static_assert( noexcept(il.data()) );
+ static_assert( noexcept(il.empty()) );
+}
+
diff --git a/libstdc++-v3/testsuite/18_support/initializer_list/range_access.cc
b/libstdc++-v3/testsuite/18_support/initializer_list/range_access.cc
index 8d7cb0dfb3f..feffd4ee4e4 100644
--- a/libstdc++-v3/testsuite/18_support/initializer_list/range_access.cc
+++ b/libstdc++-v3/testsuite/18_support/initializer_list/range_access.cc
@@ -20,16 +20,12 @@
// 18.9.3 Initializer list range access [support.initlist.range]
#include <initializer_list>
+#if __cpp_lib_initializer_list >= 202511L
+# include <iterator>
+#endif
void
test01()
-{
- std::begin({1, 2, 3});
- std::end({1, 2, 3});
-}
-
-void
-test02()
{
static constexpr std::initializer_list<int> il{1};
static_assert( std::begin(il) == il.begin() );
diff --git
a/libstdc++-v3/testsuite/18_support/initializer_list/range_access14.cc
b/libstdc++-v3/testsuite/18_support/initializer_list/range_access14.cc
new file mode 100644
index 00000000000..b98e9ccf7b5
--- /dev/null
+++ b/libstdc++-v3/testsuite/18_support/initializer_list/range_access14.cc
@@ -0,0 +1,24 @@
+// { dg-do compile { target c++14 } }
+
+#include <initializer_list>
+#include <iterator>
+
+void
+test01()
+{
+ static constexpr std::initializer_list<int> il{1, 2};
+ static_assert( std::cbegin(il) == il.begin() );
+ static_assert( std::cend(il) == il.end() );
+#if __cplusplus >= 201703L
+ static_assert( std::rbegin(il).base() == il.end() );
+ static_assert( std::rend(il).base() == il.begin() );
+ static_assert( std::rbegin(il).base() == il.end() );
+ static_assert( std::rend(il).base() == il.begin() );
+#endif
+ static_assert( noexcept(std::cbegin(il)) );
+ static_assert( noexcept(std::cend(il)) );
+ static_assert( noexcept(std::rbegin(il)) );
+ static_assert( noexcept(std::rend(il)) );
+ static_assert( noexcept(std::crbegin(il)) );
+ static_assert( noexcept(std::crend(il)) );
+}
diff --git
a/libstdc++-v3/testsuite/18_support/initializer_list/range_access14_neg.cc
b/libstdc++-v3/testsuite/18_support/initializer_list/range_access14_neg.cc
new file mode 100644
index 00000000000..e1c4062fc67
--- /dev/null
+++ b/libstdc++-v3/testsuite/18_support/initializer_list/range_access14_neg.cc
@@ -0,0 +1,18 @@
+// { dg-do compile { target c++17 } }
+
+#include <iterator>
+
+void
+test01()
+{
+ (void)std::cbegin({1, 2, 3}); // { dg-error "no matching function for call"
}
+ (void)std::cend({1, 2, 3}); // { dg-error "no matching function for call"
}
+ (void)std::rbegin({1, 2, 3}); // initializer_list overload not removed
+ (void)std::rend({1, 2, 3}); // initializer_list overload not removed
+ (void)std::crbegin({1, 2, 3}); // { dg-error "no matching function for call"
}
+ (void)std::crend({1, 2, 3}); // { dg-error "no matching function for call"
}
+
+}
+
+// { dg-prune-output "cannot bind non-const lvalue reference of type" }
+// { dg-prune-output "which is of non-class type" }
diff --git
a/libstdc++-v3/testsuite/18_support/initializer_list/range_access17.cc
b/libstdc++-v3/testsuite/18_support/initializer_list/range_access17.cc
new file mode 100644
index 00000000000..443aae3aaa0
--- /dev/null
+++ b/libstdc++-v3/testsuite/18_support/initializer_list/range_access17.cc
@@ -0,0 +1,16 @@
+// { dg-do compile { target c++17 } }
+
+#include <initializer_list>
+#include <iterator>
+
+void
+test01()
+{
+ static constexpr std::initializer_list<int> il{1};
+ static_assert( std::data(il) == il.begin() );
+ static_assert( std::size(il) == il.size() );
+ static_assert( !std::empty(il) );
+ static_assert( noexcept(std::data(il)) );
+ static_assert( noexcept(std::size(il)) );
+ static_assert( noexcept(std::empty(il)) );
+}
diff --git
a/libstdc++-v3/testsuite/18_support/initializer_list/range_access17_neg.cc
b/libstdc++-v3/testsuite/18_support/initializer_list/range_access17_neg.cc
new file mode 100644
index 00000000000..fbe18f78bf8
--- /dev/null
+++ b/libstdc++-v3/testsuite/18_support/initializer_list/range_access17_neg.cc
@@ -0,0 +1,14 @@
+// { dg-do compile { target c++17 } }
+
+#include <iterator>
+
+void
+test01()
+{
+ (void)std::data({1, 2, 3}); // { dg-error "no matching function for call"
"" { target c++26 } }
+ (void)std::size({1, 2, 3}); // uses array overload
+ (void)std::empty({1, 2, 3}); // uses array overload
+}
+
+// { dg-prune-output "cannot bind non-const lvalue reference of type" }
+// { dg-prune-output "which is of non-class type" }
diff --git
a/libstdc++-v3/testsuite/18_support/initializer_list/range_access_neg.cc
b/libstdc++-v3/testsuite/18_support/initializer_list/range_access_neg.cc
new file mode 100644
index 00000000000..e4bff4ecbed
--- /dev/null
+++ b/libstdc++-v3/testsuite/18_support/initializer_list/range_access_neg.cc
@@ -0,0 +1,13 @@
+// { dg-do compile { target c++11 } }
+
+#include <iterator>
+
+void
+test01()
+{
+ (void)std::begin({1, 2, 3}); // { dg-error "no matching function for call"
"" { target c++26 } }
+ (void)std::end({1, 2, 3}); // { dg-error "no matching function for call"
"" { target c++26 } }
+}
+
+// { dg-prune-output "cannot bind non-const lvalue reference of type" }
+// { dg-prune-output "which is of non-class type" }
--
2.54.0
